1. Field of the Invention
The present invention pertains to an exhaust assembly for use in marine outboard and inboard/outboard engines and, more particularly, to an exhaust valve arrangement for opening and closing an exhaust outlet of the exhaust assembly.
2. Discussion of the Prior Art
It is widely known in the art of marine propulsion units to form an exhaust passage inside a propeller hub. In these known arrangements, exhaust gases developed during operation of the marine propulsion unit were expelled into the water through an exhaust outlet of the exhaust passage. Since the exhaust outlet was submerge in water during operation of the watercraft, the noise generated during expulsion of the exhaust gases was minimized. However, due to the diameter of the propeller hub and the size of the casing which supports the propeller hub in such prior art arrangements, the size of the area of the exhaust outlet was limited. Limiting the area of the exhaust outlet caused exhaust back pressure to increase during periods of high engine load which resulted in a decrease in operation performance.
To overcome this problem, it has been proposed to provide an exhaust passage within a cavitation casing located above the propeller hub. By this construction, exhaust gases could be ejected rearwardly out of both the exhaust passage formed inside the propeller hub and the exhaust passage formed in the cavitation casing. Thus, the total area of the exhaust outlet was increased to effectively prevent the development of back pressure which resulted in improved engine performance. Unfortunately, the exhaust outlets provided in the cavitation casing always remained open such that under certain operating conditions, such as when the boat was planning, the exhaust outlet in the cavitation casing was located above the surface of the water which resulted in an exhaust noise problem.
The prior art has recognized this noise problem and has attempted to solve it by providing a valve to open and close the exhaust outlet. The exhaust valve according to the prior art is supported at the top of the exhaust opening and is suspended downward. With this valve arrangement, the lower edge of the valve can be engaged by various currents in the water and rotated to the open position such that the valve would substantially remain open when the boat had reached a certain speed. On the other hand, at speeds below a certain level, the exhaust valve would remain closed and thereby reduce exhaust noise. However, when the valve is opened by the outward pressure of the water currents acting on its lower edge, the lower edge of that valve is always in contact with the water surface. That same water surface also acts to close off the opening such that, during certain operating ranges of the boat, insufficient emission of the exhaust gases developed, resulting in a build up of exhaust gas back pressure and decrease of engine performance.
Therefore, there exists a need in the art for an exhaust passage arrangement for use in a marine propulsion unit which can function to allow appropriate free flow of exhaust gases so as to prevent exhaust gas back pressure throughout the entire range of operation of the marine propulsion unit.